Roosters custodian Roger Tuivasa-Sheck has surprised plenty with just how quickly he has transformed himself from fleet-footed winger to first-class fullback in his debut season in the No.1 jersey – but the seemingly effortless transition was anything but.

Tuivasa-Sheck has remodelled his body and fitness since ex-fullback and skipper Anthony Minichiello retired at the end of 2014, dropping five kilograms and turning his focus from power running to endurance running.

The change has paid off in spades with the Kiwi flyer surging to 5,270 metres in 22 games at an average of 240 metres per week; his nearest challengers are kilometres behind him (literally – Storm prop Jesse Bromwich is next best, is 2.2 kilometres away with 3,475 metres at 158 per game).

And it's not just his running game – Tuivasa-Sheck's ball-playing has grown immensely, setting up eight tries this year in addition to the 11 he's scored.

The 22-year-old said there was plenty of hard work involved – both as an understudy to Minichiello last year – and over the pre-season in adapting him to the more intense rigours of life at the back of the field compared to on the flanks.

"I'm happy I took the hard work that Mini based [his game on] and I'm happy I took that [lesson] from him and learned that from him," Tuivasa-Sheck said.

"A lot went into [the pre-season], I started from November day one, straight into my diet, straight into video work and what I needed to do to become a fullback, a lot of running, extra fitness on my own, doing a lot of testing on my own.

"We have our team sessions with a lot of mass runs, up and downs, up and downs. When that's all done I'd have a separate session, we'd call it 40-20-60 where I'd do runs on my own, just constant runs with the team making their way off the pitch and that was the extras I need to do."

Team speed coach Hayden Knowles had viewed Tuivasa-Sheck as more of a power runner when he was on the wing previously, and saw a need to adapt that to the greater distance required of a fullback.

"Hayden saw me as a power runner, always stepping through gaps, but when I got the fullback role I had to change to a more endurance runner with a bit of speed and cut down a bit of weight and keep the power that I had.

"In 2014 I think I was playing at 97, 98 [kilograms], then I dropped down to 92, 93, 94. I've been going up and down, it's pretty tough, they manage your food but it's been good so far."

The staff closely monitor his workload to the extent some days he'd come in and high performance manager Travis Touma would tell him not to do any running. However both off the field and on it, keeping a lid on Tuivasa-Sheck's running metres is easier said than done.

"I feel proud about it and it just shows the player I really wanted to become," Tuivasa-Sheck said of his transition.

"I enjoy running the ball, it happens both ways – it takes up a bit of my energy when big plays come on and that's where I need to learn to balance it, but I get so excited sometimes, I just want the ball all the time!"

With his side set to travel to bogey ground Brookvale Oval minus three big name stars – Jared Waerea-Hargeaves and Mitchell Pearce (injured) and centre Michael Jennings (suspended), to face a desperate Manly side, Tuivasa-Sheck has faith in the extended squad to get the job done.

"We've always had faith in our whole wider squad and for players like Jackson Hastings to step up and play that half role, it was awesome to see that [last week after Pearce went off], and for our bench middles to step up to take on that leader role for Jared when he went off was solid," he said.

"I like working with Jackson, he's not afraid to throw the ball around and try some new stuff so I like working with him. He's similar to Pearcey... he's got good footwork and can run the ball as well and has a good passing game. The boys have faith in him."

The Roosters need a win on Friday night to maintain the competition lead after Brisbane edged ahead with a big win over the Rabbitohs on Thursday night.